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Q-reeus
Q-reeus is offline
#13
Feb10-11, 09:37 AM
P: 1,115
Quote Quote by Ken G View Post
The timescale is much faster, the sound crossing time of a few minutes. That's the timescale for dynamical stability, and is so short that we may assume the gas is virialized...
Wow, surprised at first. But a little thought and clearly convective flow rate vs speed of sound in a fluid are vastly different. A bit like conduction charge speed in a metal vs signal speed in the same. And these sound waves can cross the convective zone with little dissipation?
...That means stars have a somewhat opposite response to being heated than you might think-- they get cooler when you add heat to them! "They plump when you cook 'em", basically, and the work done against gravity ends up lowering the temperature-- so sometimes it is said they have a "negative heat capacity," in effect. This prevents the thermal runaway that fusion might otherwise be given to.
Ah yes - core pushing out experiences a damping back-reaction, meanwhile the outflow towards the exterior enlarges the star somewhat, which requires conversion of thermal into increased gravitational potential energy. Cool (pun intended)!
Rather than an issue of damping, I think it would be an issue of an absence of an "inertia" term. Stable dynamical equilibria overshoot and generate oscillation because of inertia, which then has to damp out, but stable thermal equilibria (or bank accounts) have no such inertia-- there is no term that drives the cycle to overshoot once the balance between fusion rate and luminosity is recovered.
Well in a sense, sans some interesting learning and relearning on the way, have come full cycle re original comment - it's all in equilibrium, so what the heck. But you will probably argue it's only quasi-equilibrium - there's a usually imperceptible dynamical balance driven from without, yes?